The focus of the study will be on the effect of chronic hypoxia on respiratory control in man. Chronic hypoxemia leads to altered states of breathing in man and in animals at high altitude. The high-altitude residents, however, differ from the newly acclimatized sojourners in that they show "blunted" ventilatory responses to hypoxia. The sojourners are known to show periodic breathing and apnea particularly during sleep. There is reason to hypothesize that the altered state of control of breathing in the high altitude natives might prevent instability of breathing at high altitude. By the same reasoning these individuals would differ from the newcomers in their responses to withdrawal of the hypoxic stimulus. The phenomena of periodic breathing and the "residual" effect of chronic and semi-chronic hypoxia are not self-consistent. Sleep, however, might make radical changes in the control system, and eliminate the "residual" drive. The object of the study is to examine control of breathing in the high-altitude natives in particular during states of sleep, awakeness and exercise. It has been proposed to use acetazolamide to alter the internal environment with respect to CO2 and O2 transport, and investigate the peripheral and central chemoreflex components of respiratory control. The basic measurements will consist of respiratory flow and volume, alveolar PO2 and PCO2, and arterial O2 saturation during various experimental procedures.